Because life is divided by kind. This concept arose long before modern man. A horse, a zebra, and a mule are all of the horse kind. They are capable of interbreeding. African, Caucasian, and Oriental persons are variety of one kind - the human kind, and are capable of reproduction. Animals of one kind can interbreed. But not even all varieties within a kind are capable of rinterbreding. This is especially true of fish. However, no two animals of differnt kind are capable of interbreeding. A horse cannot mate with an alpaka and yield a new kind. Kinds can be specified into speices if you like. Many species could be said to be of one kind. A fish cannot interbreed with a lizard. A mouse cannot interbreed with a gerble, even though they are of the same kind - rodent kind! What is never found however, are two of different kinds capable of interbreeding. Life is divided by kind. This is in fact why it was possible for the first man to name all of the kinds in a day as God presented them to him. Adam did not have to name the individual species, only the kinds. This cuts the task down al;most immesurably.